At a coal tar plant, coal tar distillation gives distillates such as carbolic oil, naphthalene oil, wash oil and anthracene oil. When subjected to phenols(or acid)-extraction or base-extraction treatment or when used for recovery of light oil from coke over gas, such distillates are in contact with water and thereby yield a water-containing waste oil. Further, coal tar contains water in itself, and therefore, a waste oil likewise forms during its storage in a tank or at the time of its distillation. Furthermore, similar waste oils result from the washing of the tanks for various distillate oils.
Oil originated from coal tar have greater hydrophilic property than oils originated from petroleum, and accordingly they tend to form a water-in-oil emulsion. Depending on their origins, water-containing waste oils differ more or less from each other in their characteristics. However, they normally contain not only oil and water but also solid substance, i.e. sludge, and they form an extremely stable emulsion comprising these three components.
Water-containing waste oils collected from coal tar plants have a relatively uniform composition and they normally contain 10 to 40% by weight of oil, 1 to 5% by weight of sludge and the rest being water. The sludge contains inorganic substances mainly comprising iron compounds, resinous matters comprising aromatic condensed ring compounds, coke powder, coal powder, etc., and is swelled in a water-containing waste oil about ten times of the volume of its dried state. The oil fraction contains mainly benzene homologues as light distillates, naphthalenes as medium distillates and tricyclic aromatic compounds such as anthracenes as heavy distillates. The specific gravity of the oil fraction is relatively close to that of water and it varies depending upon the composition of the particular oil. The specific gravity becomes smaller than water as the proportion of light distillates increases or as the temperature rises.
Further, various petroleum-based water-containing waste oils are known such as rolling oil wastes, lubricating oil wastes, or wash oil wastes which result from the rolling or tube making operations at ironworks, or from the washing of a coke oven gas with a petroleum-type absorption oil, or from machine tool works; crude oil wastes, heavy oil wastes, residual oils from tanks, sludge oils resulting from washing of crude oil tanks or heavy oil tanks, or the production line of lubricating oils at petroleum refineries and petrochemical factories; lubricating oil wastes, automobile engine oil wastes, cutting oil wastes, anti-corrosive oil wastes from automobile industries, machine making factories and ship yards. Depending upon their origins, these petroleum-based water-containing waste oils differ more or less in their characteristics. However, they normally contain not only oil and water but also sludge as solid constituent, and they also contain additives such as emulsifiers. Thus, they form an extremely stable emulsion. For instance, waste oils derived from the rolling operation at a metal working plant, or from the washing operation of coke oven gas or from the machine tool works, are in a form of a water-in-oil emulsion (hereinafter referred to as W/O emulsion) comprising mixed oil which contains as principal constituent a petroleum-based oil such as a rolling oil, lubricating oil, machine oil or wash oil, water and fine solid particles such as iron oxides or carbon particles. Likewise, waste oils or sludge oils derived from the washing operation of the crude oil or heavy oil tanks at a petroleum refinery, or waste oils derived from the machine making factories or the like, are water-containing waste oils in a form of W/O emulsion.
These water-containing waste oils normally comprise at least 50% by weight of petroleum-based oil, 0.1 to 1.0% by weight of solid constituent (dry basis) and the rest being water. By a petroleum-based oil is meant an oil containing non-aromatic oil components as its principal constituents. The solid constituent comprises metal powders such as iron powder, iron compounds such as iron oxides, carbon particles, dusts or sands. In combination with water or oil, the solid content forms a sludge.
It is extremely difficult to treat such tar-based or petroleum-based water-containing waste oils for separation even by subjecting them directly to a centrifugal separator, as they are stable and undergo no substantial change with time in their original proportion of oil, emulsion, water and solid fractions. Further, the water-containing waste oils have poor combustibility and therefore they are inferior as fuels and they tend to block up the burner tip with sludge components.
It is known to add a certain surface active agent in order to recover oil components from tar-based water-containing waste oils (Japanese Unexamined Patent Publication No. 96,785/1976). However, this method is intended primarily to treat oil components heavier than water. Accordingly, when subjected to the centrifugal operation, while water can be separated satisfactorily, the separation of the sludge and oil can not be accomplished to a satisfactory degree. No satisfactory results are obtainable in such case where recovery of high quality oil is desired and where waste oils containing oil components lighter than water are treated.
There have been proposed various methods for treating petroleum-based emulsion type water-containing waste oils such as those mentioned above, for instance, a method wherein the waste oils are heated under pressure, and then cooled to separate the three phase of oil, water and solid, and each phase is centrifugally separated (Japanese Patent Publication No. 47,722/1976), a method wherein a waste oil is heated to demulsify the emulsion and after cooling, each layer is separated (Japanese Patent Publication No. 43,305/1976) or a method wherein a waste oil is centrifuged while being heated, whereby the sludge is separated from the oil and water, and the oil and water thus obtained are further centrifuged for separation (Japanese Unexamined Patent Publication 123,403/1977). These methods require great amounts of energy for heating, pressurizing and centrifuging and yet the separation of the oil and sludge is inadequate. Further, there has also been proposed a method wherein a light oil fraction is added to a sludge oil to extract the oil component and the extracted sludge oil thus obtained is heat treated or treated with a coagulant and then centrifugally separated into water and solid components (Japanese Patent Publication 15,025/1972), a method wherein a hydrocarbon solvent containing as its principal constituents propane, butane or a hydrocarbon mixture containing from propane to a light oil, is added to a waste oil or sludge thereby separating it into oil and sludge components (U.S. Pat. No. 3,684,699 and Japanese Unexamined Patent Publication No. 39,601/1974). However, these methods have drawbacks such that the operation is rather complicated or the separation of the oil and sludge components is inadequate. Further, it is known to add a coal tar neutral oil containing a small amount of a surface active agent dissolved therein to a sludge so as to disperse the sludge (U.S. Pat. No. 2,559,574). However, this method is not intended to separate the oil and water.
Accordingly an object of the present invention is to provide a novel method for treating a water-containing waste oil comprising oil, water and sludge and which is in a form of a water-in-oil emulsion.
Another object of the invention is to provide a method wherein a certain treating oil is added to a water-containing waste oil originated from coal tar or petroleum thereby to recover the oil components from the waste oil.